Cross-linking, physically or covalently, is the fundamental and necessary attribute to introduce a 3D network in a polymer. Conventional bifunctional cross-linkers (e.g., bisacrylamide (Schild, Prog. Polym. Sci., 17:163-249 (1992))) are inactive after the formation of the covalent crosslinks during the polymerization.
Whereas most cross-linkers in synthetic networks are less active, cross-linkers in biopolymer networks are active. For example, the cytoskeleton of muscle cells has myosin motors as the active cross-linkers to crosslink actin filaments (Rayment et al., Science, 261:58-65 (1993); Harada et al., Nature 326:805-808 (1987)). Marveled by this amazing machinery evolution of converting chemical energy to mechanical motion, researchers have spent considerable efforts on using actin and myosins to create active gels in vitro to explore the structures and dynamics of these minimal active networks (Kohler et al., Nat. Mater., 10:462-468 (2011); Boal et al., Small, 2:793-803 (2006); Banerjee et al., Soft Matter, 7:463-473 (2011); Chen et al., Soft Matter, 7:355-358 (2011); Tsuchiya et al., Angew. Chem.-Int. Edit. 49:724-727 (2010)).
Although active or functional molecules are increasingly used for making supramolecular gels (Zhang et al., Angew Chem., 124:7117-7121 (2012); Kohsaka et al., Angew. Chem.-Int. Edit., 50:4872-4875 (2011); Tamesue et al., Angew. Chem.-Int. Edit., 49:7461-7464 (2010); Kretschmann et al., Angew. Chem.-Int. Edit., 45:4361-4365 (2006); Oku et al., Angew. Chem.-Int. Edit., 43:966-969 (2004)) such as self-healing soft materials (Zhang et al., Agnew Chem., 124:7117-7121 (2012); Imato et al., Angew. Chem.-Int. Edit., 51:1138-1142 (2012); Amamoto et al., Angew. Chem.-Int. Edit., 50:1660-1663(2011); Wang et al., Nature, 463:339-343 (2010)), synthetic polymeric gels with active cross-linkers have not been generated.
[Ru(bipy)3]n+ is as a well-known coordination compound (Balzani et al., Chem. Rev., 96:759-833 (1996); Crosby et al., J. Chem. Phys., 43:1498 (1965); Caspar et al., J. Am. Chem. Soc., 105:5583-5590 (1983); Maness et al., J. Am. Chem. Soc., 118:10609-10616 (1996)) and has long lifetime of excited state that has both oxidizing and reducing properties. Its derivatives have served as photosensitizers (Demas et al., J. Am. Chem. Soc., 93:1800 et seq. (1971); Meyer, Accounts Chem. Res., 22:163-170 (1989); Kalyanasundaram et al., Coord. Chem. Rev., 177:347-414 (1998)), electrochemiluminescent components (Tokel et al., J. Am. Chem. Soc., 94:2862 et seq. (1972)), the core of a bacteria sensing element (Grunstein et al., J. Am. Chem. Soc., 133:13957-13966 (2011)), a component for conducting polymers (Zhu et al., J. Mater. Chem., 9:2123-2131 (1999)), and the cores of star polymers (Lamba et al., J. Am. Chem. Soc., 119:1801-1802 (1997); Collins et al., Macromolecules, 31:6715-6717 (1998); McAlvin et al., Macromolecules, 32:6925-6932 (1999)).
[Ru(bipy)3]n+ also serves as a redox catalyst for a well-established chemical oscillator, namely the Belousov-Zhabotinsky (BZ) reaction (Bansagi et al., Science, 331:1309-1312 (2011); Noszticzius et al., J. Am. Chem. Soc., 101:3177-3182 (1979); Maselko et al., J. Chem. Phys., 85:6430-6441 (1986); Vanag et al., Phys. Rev. Lett., 86:552-555 (2001); Vanag et al., Phys. Rev. Lett., 87:228301 (2001); Zeyer et al., J. Phys. Chem. A, 102:9702-9709 (1998); Bolletta et al., J. Am. Chem. Soc., 104:4250-4251 (1982)). The redox switch of [Ru(bipy)3]n+ complex has led to the development of a self-oscillating gel that swells in oxidized state (Ru(III)) and shrinks in reduced state (Ru(II)) during the BZ reaction (Yoshida et al., J. Am. Chem. Soc., 118:5134-5135 (1996)). However, in that type of gel, N,N′-methylenebisacrylamide (BIS) (Yoshida, Adv. Mater., 22:3463-3483 (2010)) is the cross-linker and the ruthenium(II/III) complex is a pendant of the cross-linked chains of poly(NIPAAm) (Schild, Prog. Polym. Sci., 17:163-249 (1992); Chen et al., Nature, 373:49-52 (1995); Eliassaf, J. Appl. Polym. Sci., 22:873-874 (1978); Park et al., Biotechno. Prog., 10:82-86 (1994).
Active polymers have broad technological applications. However, there are limited methods to produce such materials except memory alloys and temperature responsive gels, where the polymer subunits of the gels are responsive to temperature. The present invention overcomes these and other deficiencies in the art.